Discharge lamps used for vehicle headlights are disclosed in JP-B2 3596812 (Patent Registration) (hereinafter referred to as Patent Reference 1) and JP-A 2007-179998 (KOKAI) (hereinafter referred to as Patent Reference 2). The discharge lamps have a double tube structure of an inner tube and an outer tube attached to cover the inner tube. The inner tube has a light-emitting unit in which a rare gas and a metal halide are enclosed, and seal portions which are formed at either end of a luminous tube, and seal a metal foil and an electrode.
It is known that this type of discharge lamp is started with difficulty because a voltage of several kV to several tens of kV is required to start the lamp. Patent Reference 1 discloses that a gas capable of inducing a dielectric barrier discharge is enclosed into a space comprising an inner tube and an outer tube, and the dielectric barrier discharge is induced at start up to decrease a starting voltage, thereby enabling to start readily.
Patent Reference 1: JP-B2 3596812 (Patent Registration)
Patent Reference 2: JP-A 2007-179998 (KOKAI)